1. Field of the Invention
The present invention relates generally to high sulfur content polymeric materials and composites, methods for making them, and devices using them such as electrochemical cells and optical elements.
2. Technical Background
Elemental sulfur is produced in vast quantities (millions of tons annually) by the petrochemical industry as a by-product from the hydrodesulfurization of oil. Current industrial utilization of elemental sulfur is centered around sulfuric acid, agrochemicals, and vulcanization of rubber. Nonetheless, abundant sulfur remains and is stockpiled. While sulfur feedstocks are plentiful, sulfur is difficult to process. S8, for example, is a brittle, intractable, crystalline solid having poor solid state mechanical properties, poor solution processing characteristics, and a limited slate of synthetic methodologies developed for it.
Elemental sulfur has been explored for use in lithium-sulfur electrochemical cells. Sulfur can oxidize lithium when configured appropriately in an electrochemical cell, and is known to be a very high energy density cathode material. However, electrical limitations of pure elemental sulfur, such as low cycle stability and poor conductivity) have limited the development of this technology. For example, one key limitation of lithium-sulfur technology is the ability to retain high charge capacity for extended cycle lifetimes. Cells based on current lithium ion technology has low capacity (180 mAh/g) but can be cycled for 500-1000 cycles. Lithium-sulfur cells based on elemental sulfur have very high initial charge capacity (in excess of 1200 mAh/g, but their capacity drops to below 400 mAh/g within the first 100-500 cycles. Thus, materials that can provide extended cycle lifetime while retaining reasonable charge capacity are desired.
There have been several recent attempts to form sulfur into nanomaterials for use as cathodes in lithium-sulfur electrochemical cells, such as impregnation into mesoporous carbon materials, encapsulation with graphenes, encapsulation into carbon spheres, and encapsulation into conjugated polymer spheres. While these examples demonstrate that the encapsulation of elemental sulfur with a conductive colloidal shell in a core/shell colloid can enhance electrochemical stability, these synthetic methods are challenging to implement for industrial scale performance. Hence, a new family of inexpensive, but functional materials obtained by practical methods is desirable.